In the view of Babendreier et al. (2006), the purpose of host range testing
is to determine the ecological host range of the natural enemy. This seems
ambitious, and they acknowledge that designing tests for this purpose is
difficult. An alternative view is that host range testing provides an
opportunity to define the physiological or fundamental host range, and by the
use of a range of test designs, to provide additional information with which to
predict the likely field host range. This approach seems more likely to be
successful. A test design array should take into account the following
principles:

Factors within test designs, such as the size of the arena or the presence
of plant material, may either reduce or increase the acceptability of hosts to
natural enemies (Babendreier et al. 2005). At least one test (usually a simple,
well controlled 'no choice' test in conditions that are highly suitable for the
agent) must create a 'maximum challenge' to the susceptibility of the host to
create a reliable physiological host range
(Hill 1999, Withers and Barton-Browne 2004). However, it is important for reviewers to distinguish the physiological
host range revealed by such tests from the likely field host range.

'Choice' tests are often used because this design represents field
conditions more realistically than 'no choice' tests. The value of 'choice'
tests is debated (Barratt et al. 2006), and depending on factors such as agent
size and mobility, 'no choice' may well be the more realistic design
(Hill 1999).

Natural enemies can respond differently to their host depending on age and
physiological state. A newly emerged natural enemy may have a different
intensity of response than an aged individual. For example,
Kitt and Keller (1998) found that older Aphidius rosae adults were
more selective than newly
emerged individuals. A satiated agent may attack fewer hosts than a deprived
agent. The duration of a test may therefore influence its outcome. Test designs
must consider these possibilities. The issues posed by time and
behaviour-dependent effects are reviewed by Withers and Barton-Browne (2004).

Marohasy (1998) reviewed the design and interpretation of host-specificity
tests for weed biological control agents and recommended that tests should be
designed to take more account of agent behaviour. She suggested methods for
measuring the rank order of acceptability of test species, and the time taken
before an agent will accept a lower-ranked plant species as additional measures
of host acceptance. These novel approaches may well have application in the
testing of arthropod control agents.

In their native range the host range of parasitoids and phytophages can vary
between populations (see discussion about biotypic and geographic variation
above). Best practice in New Zealand now dictates that only those populations
that have been tested should be released. Later releases of populations from
other sources should be subject to confirmation of that host range by limited
testing (Withers et al. 2008).

van Lenteren et al. (2006) have summarised the issues that should be
considered when designing host specificity tests on arthropod natural enemies,
and propose a methodology for addressing those issues. Their methodology is very
clear and well-annotated, and provides a sound baseline for the development of a
testing regime. However, they acknowledge that a theoretical and methodological
background for such a testing regime for arthropod natural enemies is not yet
mature. They summarise what information should be brought to bear on a test
design:

know the foraging behaviour of the natural enemy;

know the influence of quality and rearing conditions of the host plant, host
and natural enemy. These should be described in detail to trace the effects of
conditioning, learning and multitrophic communication on results;